This invention relates to the field of circuit breakers for protection of electrical circuits, and in particular to those having adjustment means to vary certain performance characteristics of the breaker, such as its ampere rating, trip times and the like.
Circuit breakers have been in use for many years to protect electrical circuits in homes as well as in commercial and industrial establishments, and wherever electricity is used. The typical circuit breaker includes a mechanical tripping mechanism and magnetic or thermal triggering means to open the circuit on occurrence of an overcurrent fault of a pre-selected magnitude. The tripping level of such circuit breakers is usually set by the manufacturer and either cannot be changed by the user or if adjustment of the trip level is provided it can only be done within a relatively limited range and without knowing precisely what the change is. Similarly, the ampere rating and frame size of each prior art breaker is established by the manufacturer, so a circuit breaker of one frame size for an electrical distribution system of a given ampere and voltage rating could not be properly interchanged for use in an electrical system of different ampere and voltage ratings. The time between occurrence of a fault and tripping of the breaker is typically determined by such things as the size and material of the bimetal element in thermal trip mechanisms, and by such things as the rating of the trip coil and the like in magnetic trip mechanisms. The trip time delay could be varied somewhat in such prior art breakers by such things as moving the thermal trip bimetal closer to or further from the delatching member and the like, varying the magnetic gap in the magnetic trip mechanisms, and so on. Such changes however were within a limited range, and the amount of change could not be determined easily and precisely. Significant changes in trip times would often require changing of the components themselves such as the bimetal element, trip coil, and the like.
It is desirable to provide a circuit breaker which has greater flexibility, and in which the ampere ratings, pick-up levels, trip time delay, and the like can all be varied easily and precisely to enable use of a single breaker in a wide variety of electrical distribution systems. For example, when used as a main circuit breaker in a distribution system having a number of branches fed by separate branch circuit breakers, it would be desirable to be able to adjust the pick-up level and time delay of the main breaker to a precise point at which the downstream branch breakers are able to clear low level faults but at which the main breaker will interrupt higher level faults which would otherwise damage the distribution system or result in a fire or explosion.
The circuit breaker in accordance with the present invention enables precise adjustment of such characteristics of ampere rating, pick-up level, time delay and the like by combining an electronic fault sensing and trip initiating unit with a magnetic and mechanical tripping mechanism. Such electronic, magnetic and mechanical combination facilitates inclusion of ground fault capability and shunt trip capability, which are normally higher cost accessories when added to prior art circuit breakers.